Assembly for detecting the superficial structures of fingers and/or palms

ABSTRACT

An arrangement for detecting the surface structures of fingers, palms or other parts of the skin placed on an image scanning surface, wherein the image scanning surface is formed at a scanning prism, an illumination beam path is directed to the image scanning surface in the interior of the scanning prism, and the light reflected from the latter impinges on the reception surface of an optoelectronic detection device. In an arrangement of the type described above, a part of the illumination beam path or a separate illumination beam path is directed to reference structures which are fixed in place and, from the latter, to the reception surface, wherein separate partial areas of the reception surface are reserved for the detection of the surface structures and for the detection of reference structures.

BACKGROUND OF THE INVENTION

a). Field of the Invention

The invention is directed to an arrangement for detecting the surfacestructures of fingers, palms or other parts of the skin placed on animage scanning surface, wherein the image scanning surface is formed ata scanning prism, an illumination beam path is directed to the imagescanning surface in the interior of the scanning prism, and the lightreflected from the image scanning surface impinges on a receptionsurface of an optoelectronic detection device.

b). Description of the Related Art

As with other precision optical equipment in which image information isacquired by scanning, arrangements of this type must be monitored andcalibrated by technical apparatus to ensure operability and accuracyrequirements.

For example, in a line image scanner outfitted with an image scanningsurface, a sensor line and a mechanical drive for displacing the sensorline, it is provided in Patent DE 41 03 646 C1 to carry out thegeometric calibration with the aid of reference marks which are appliedto an edge of the image scanning surface running vertical to the sensorline and to image them on the sensor line during the image scanning. Theimages of reference marks which are obtained in the course of a scan areused to monitor and, if necessary, correct the displacement speeds and,in this way, to compensate for distortions caused by a displacementspeed diverging from the reference value. However, this suggestion isnot suited to calibration of devices with a stationary surface imagescanner.

In devices having a stationary surface image scanner, for example, anarray of individual sensors, it was formerly common for referencestructures which are used over and over again in the course of operationas a basis for comparison with the results obtained by the samearrangement to be included with delivery to the user. The user orservice can ascertain from time to time on the basis of these referencestructures whether the quality parameters have been maintained or haveresulted in deviations due to influences of some kind, e.g., aging orwear of subassemblies, which must be corrected. It is also oftennecessary to check the functioning of the equipment after assembly,e.g., for cleaning purposes.

The reference structures in the form of test structures are arranged ona carrier, e.g., a sheet or foil, which is placed on the image scanningsurface for checking the equipment function. The test structures arerecorded by the device and the results of the recording are subsequentlycompared with the test structure. Based on this comparison, it ispossible to ascertain whether the arrangement still corresponds toquality requirements or whether changes have taken place. The foil withthe test structures is then removed from the image scanning surfacewhich is now available again for detection of surface structures offingers and palms.

In this regard, the same areas of the image scanning surface areutilized for equipment testing and for the detection of the surfacestructures of the fingers and palms. In order to preclude falsificationof test results, it is necessary to thoroughly clean the image scanningsurface before placing the foil. Further, the foil must be carefullystored and handled so that it is not damaged, and it must also becarefully placed so that there is optical contact with the imagescanning surface. In this connection, it is often recommended that anoptical coupling medium, such as oil or water, is introduced between theimage scanning surface and the foil. However, even when this is done,there is also a danger of falsified results, namely, when the layerthicknesses between the image scanning surface and the foil are toogreat or too varied.

The above-described procedure for calibration is stipulated, forexample, by the Federal Bureau of Investigation (FBI) in the USA forautomatic fingerprint identification systems which are used for officialdetection purposes to ensure that these systems meet the requiredtechnical parameters over their entire period of use.

OBJECT AND SUMMARY OF THE INVENTION

Based on this prior art, it is the primary object of the invention toreduce the sources of error and the possibilities of error when checkingarrangements for detecting the surface structures of fingers and palmssupported on an image scanning surface.

According to the invention, in arrangements of the type described above,a part of the illumination beam path or a separate illumination beampath is directed to reference structures and, from the latter, to thereception surface of the optoelectronic detection device, whereinseparate partial areas of the reception surface are reserved for thedetection of surface structures and for the detection of referencestructures.

By means of the arrangement according to the invention, it is possibleat desired times before, during or after the recording of fingerprintimages or palm print images to obtain images of the referencestructures, to base a check of the quality parameters on the results,and to draw conclusions from these results about the optical andelectronic characteristics of the arrangement.

In a preferred construction of the invention, it is provided that thereference structures are arranged on the image scanning surface so thatthey cannot change position, wherein separate partial areas of the imagescanning surface which do not intersect are reserved for the referencestructures and for the support of the fingers or palms and a commonillumination beam path is provided for these partial areas. The areas ofthe image scanning surface which are provided for placement of thefingers or palms are accordingly no longer required for carrying out theequipment test.

A substantial advantage consists in that the detection of fingerprintsand hand prints and the recording of images of the reference structuresare no longer mutually exclusive and, therefore, can be carried outsimultaneously and not only consecutively. Further, a falsification ofthe test results due to contamination caused by contact with the fingersor palms, at least for the areas of the image scanning surface reservedfor the reference structures, and the occurrence of errors resultingfrom the arrangement of optical intermediate layers (oil, water) betweenthe image scanning surface and a movable carrier provided with thereference structures is ruled out. The disadvantages of the prior artare overcome in this way.

In the latter constructional variant, the reference structures areapplied directly to the image scanning surface, i.e., they are arrangedin the object plane. In contrast, in a further development of theinvention, the reference structures, although they are likewiseunchangeable with respect to position, are positioned outside of theobject plane, for example, at the entrance face for the illuminationbeam path and/or at the exit face for the beam reflected by the imagescanning surface. Accordingly, the reference structures are placed outof contact with the fingers or palms to be imaged in a highly dependablemanner.

Further, the optoelectronic detection device is combined with anevaluating circuit in which the images which are obtained from thereference structures and which characterize the actual state of thearrangement are subjected to a comparison with electronically storeddata giving the reference state, and conclusions are drawn concerningwhether or not the quality parameters specific to the equipment arebeing adhered to. Automatic monitoring is possible in this way.

In this regard, it lies within the scope of the invention to provide acontrol circuit by which, for example, whenever the apparatus is putinto operation and/or after expiration of a predetermined operatingperiod, the reference structures are recorded and the recorded imagesare compared with the stored data. When this comparison has negativeresults, the erroneous detection of fingerprints or hand prints isprevented when minimum requirements are not met by triggering an alarm,an error message on the device display or by emitting a switch-off pulseto the device control.

In addition or as an alternative to the arrangement described above,means are provided for manually triggering the optical detection of thereference structures and checking can be initiated at optionaldetermined times. For this purpose, for example, there is a hand switchcombined with the control circuit, which hand switch is actuated before,during or after detection of fingerprints and hand prints.

Further, a cover can be provided over the reference structures arrangedon the image scanning surface. This prevents the reference structuresfrom exposure to continual contact by fingers or hands and accordinglyfrom contamination.

As an alternative or in addition to the stationary arrangement of thereference structures at the image scanning surface, the light entrancesurface or light exit surface at the scanning prism, it is possible toprovide an optic device for reflecting and/or mixing referencestructures into the illumination beam path, so that image informationabout reference structures reaches the reception surface of theoptoelectronic detection device in this way and can serve as a basis forthe calibration of the arrangement in the manner described above.

For example, grooves with different widths and different spacing andorientation can be used as reference marks. The contrast transfer(Contrast Transfer Function —CTF) which is used as a basis for theassessment of the modulation transfer (Modulation Transfer Function—MTF)can be determined in this way in particular. It is also possible toprovide structures with high-contrast light-dark and dark-lighttransitions in horizontal and vertical direction, so that the MTF can bechecked by Fourier analysis.

It is also possible to provide an empty field under the referencestructures which is used for checking the linearity and noise of thearrangement by adjusting the shutter of the detection device in definedsteps under constant illumination and comparing the resulting outputsignal with the data which can be called up from the memory.

It is further possible to provide two test marks which are arranged at adefined distance from one another and which are used for checking theimage scale. Accordingly, it is also possible to check the image scaleover time intervals in that the respective measurement results are usedas a basis for the comparison with values stored during manufacture,with reference to the two extension directions of the image scanningsurface.

In addition to the solution described above, the object of the inventionis further met in that semi-transparent reference structures areintroduced in the illumination beam path, on the object surface and/orin the beam reflected by the image scanning surface; in so doing, thesurface structure (of the finger or palm) to be detected as well as thesemi-transparent reference structures are acted upon by the beam and thebeam striking the reception surface of the detection device carriesimage information concerning the surface structure as well as imageinformation about the reference structures.

The reference structures can be positioned in such a way that they arecovered by the surface structure to be detected, i.e., there are noseparate areas of the beam path or of the reception surface reservedexclusively for the detection of the surface structure or exclusivelyfor the detection of the reference structures.

However, it is provided for this purpose that the evaluating circuit isconstructed according to claim 6. In this respect, a first computingfunction for determining a correction factor i_(ij) for every imageelement ij is provided in the evaluating circuit, where k_(ij)=referencegray value/actual gray value, wherein every image element ij correspondsto an individual sensor of the reception surface. The reference grayvalues are advantageously determined for every individual sensor with auniformly bright image without structures and a second computingfunction is provided by which the determined actual gray value of everyimage element ij is multiplied by the associated correction factork_(ij).

By means of this arrangement according to the invention it is likewiseachieved that checking or calibration can be carried out with fingers orpalms placed on the image scanning surface as well as when the imagescanning surface is empty. Namely, with the evaluating circuit which isconstructed as shown, the image recorded from a surface structure can beprocessed in such a way that the gray value determined for each imageelement ij is corrected such that an image is obtained which has onlythe structures of the finger or palm in the areas in which the finger orpalm is placed and the reference structure has been “corrected out”. Forthis purpose, it is assumed that the correction values k_(ij) are storedso as to enable a unique correlation to the corresponding image elementij.

When a test run is carried out without a finger or palm, a uniformlybright image without structures results with the correct correctionvalues k. If this is not the case, the correction values k_(ij) must beread in again.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described more fully in the following withreference to an embodiment example. In the accompanying drawings:

FIG. 1 shows a schematic view of a comparable arrangement according tothe known prior art;

FIG. 2 shows a schematic view of the arrangement according to theinvention;

FIG. 3 is a top view showing the division of the image scanning surface;

FIG. 4 shows the positioning of reference structures in a first variant;and

FIG. 5 shows examples for reference structures.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an arrangement for the recording of fingerprints as isknown from the prior art. In this case, a finger 3 whose print is to bedetected is placed on the image scanning surface 1 of a scanning prism2. The scanning prism 2 has an entrance face 4 for an illumination beampath 5 and an exit face 6 for the light, referred to hereinafter asobject beam path 7, reflected inside the scanning prism 2 from the imagescanning surface 1.

The illumination beam path 5 proceeds from an illumination source 8which is constructed, for example, as a two-dimensional arrangement of aplurality of light emitting diodes (LEDs) followed by a diffusor. Theillumination beam path 5 is directed to the image scanning surface 1 inthe interior of the scanning prism 2 at an angle α which makes itpossible to obtain images of the fingerprint using the principle offrustrated total reflection.

The object beam path 7 is directed through a correcting prism 9 and anobjective 10 onto the photosensitive reception surface 11 of anoptoelectronic detection device 12. The reception surface 11 is formed,for example, of a plurality of individual senors arranged in anorthogonal grid and the optoelectronic detection device 12 is part of aCCD camera which is combined, in a manner known per se, with anevaluating circuit for the electronic image information that can betapped at its output.

It can further be seen from FIG. 1 that only an area with width a of theimage scanning surface 1 is provided for placement of the finger (orseveral fingers or a palm), while the edge areas are not used. Also,only that area of the reception surface 11 having width c in the drawingplane of FIG. 1 and on which the object beam path 7 is focussed is used.

In order to carry out calibration of the arrangement, it is provided, ascan be seen in FIG. 2, that reference structures 13 are arranged in theedge areas a₁ and a₂ of the image scanning surface 1 which are locatedoutside the width a provided for the detection of fingerprints and palmprints. Beam components 5.1 and 5.2 of the illumination beam path 5strike these reference structures 13, and the beam components 5.1 and5.2 of the illumination beam which are reflected by the edge areas a₁and a₂ reach the reception surface 11 as beam components 7.1 and 7.2 ofthe object beam path 7, where they impinge on edge areas designated byc₁ and c₂. Edge areas c₁ and c₂ are located outside of the width creserved for the detection of fingerprint and palm print images.

As is shown in the following, the reference structures 13 areconstructed in such a way that their images which are received by theoptoelectronic detection device 12 and sent to the evaluating circuitcan be subjected to a comparison with electronically stored data whichserve as a measurement for the quality parameters of the arrangement andconclusions may be drawn as a result of this comparison about whetherthese quality parameters are adhered to or diverged from.

It can be seen from FIG. 3 which shows a top view of the image scanningsurface 1 that the surface portion determined by a and b is provided forrecording fingerprint or palm print images, while edge areas a₁, a₂, b₁and b₂ are reserved for the arrangement of reference structures. Just asthe edge areas a₁ and a₂ of the image scanning surface 1 correspond tothe edge areas c₁ and c₂ of the reception surface 11, edge areas arealso allocated to the edge areas b₁ and b₂ of the image scanning surface1 on the reception surface 11; however, these edge areas are only shownin their lateral projection in FIG. 2 because they lie outside of thedrawing plane and extend parallel to the latter.

FIG. 5 shows how the reference structures 13 can be formed. Structures13. 1, 13.2 and 13.3 which are formed of a plurality of lines extendingparallel to one another and arranged in groups at right angles to oneanother are used to measure contrast, wherein the measurement resultsare compared horizontally and vertically with the stored requirementsfor the MTF, e.g., at 10, 8 and 6 LP/mm. In addition, the imagesobtained from structures 13.4 serve as a basis for the comparison withthe MTF requirements for 1 LP/mm. The determined contrast values are ameasurement for the MTF which can be achieved with the arrangement.

With reference to structures 13.4, it is also possible to check theimage scale at close range by measuring the center distance of thelines, wherein, for example, the spatial frequency of 1 LP/mm is used asa basis.

Referring to structure 13.5, the MTF can be determined by means ofFourier analysis of the light-dark and dark-light transitions inhorizontal and vertical direction and can be compared with the storedreference values for these transitions.

The center distance of the structures 13.6 is known very exactly. Bycounting the image points between the centers of these structures 13.6and forming the ratio to the center distance, the image scale and thegeometric distortion are determined.

A linearity check can be based on an empty field 13.7 insofar as theshutter of the detection device is adjusted in defined steps duringconstant illumination and the respective available output signal iscompared with stored presets.

Finally, the presence of all gray steps is possible with a structure13.8. The inclination of the lines relative to the lines of the rest ofthe structures is selected in such a way that all gray steps occur andit is therefore ensured that they will be checked.

It is possible to check the signal-to-noise ratio using structure 13.7in that the spatial noise is determined in a sufficiently large surfaceportion and the temporal noise is determined at selected individualsensors of the reception surface 11 and the values obtained in this wayare checked as to whether or not they adhere to permissible deviationscompared with the stored presets.

An output signal which depends on the adherence to or deviation from thepredetermined parameters and which is available at the evaluatingcircuit can be utilized for initiating an acoustic, optic or otherindication perceptible to the operator.

In constructions of the invention according to FIG. 4, the referencestructures 13 are not arranged on the image scanning surface 1, or onlyin part; instead, they are arranged on the entrance face 4. In so doing,the reference structures 13 are positioned in portions of the entranceface 4 which lie outside of the area by which the light required forrecording the fingerprints and palm prints enters the scanning prism 2.

Alternatively, reference structures 13 can also be arranged on the exitface 6 of the scanning prism 2. The advantage in positioning on theentrance face 4 and exit face 6 consists in that the referencestructures 13 are protected against contact with the fingers or hands tobe placed on the image scanning surface 1 and accordingly fromcontamination and falsification of the reference signal. As analternative to the latter construction, protective coverings 14 can beprovided over the reference structures arranged on the image scanningsurface 1, as is shown schematically in FIG. 2.

What is claimed is:
 1. An arrangement for detecting the surfacestructures of fingers, palms or other parts of the skin placed on animage scanning surface comprising: said image scanning surface beingformed at a scanning prism; an illumination beam path being directed tothe image scanning surface in the interior of the scanning prism, andthe beam reflected from the latter impinging on the reception surface ofan optoelectronic detection device; a part of the illumination beam pathor a separate illumination beam path being directed to referencestructures and, from the latter, to the reception surface; separatepartial areas of the reception surface being reserved for the detectionof the surface structures and for the detection of reference structures.2. The arrangement according to claim 1, wherein the referencestructures are arranged so as to be fixed in place on the scanningprism, wherein separate partial areas which do not intersect arereserved for the reference structures and for the surface structures tobe detected, and a common illumination beam path is provided for thesepartial areas.
 3. The arrangement according to claim 1, wherein thereference structures are arranged so as to be fixed in place on theentrance face of the scanning prism, wherein separate partial areaswhich do not intersect are reserved for the reference structures and forthe surface structures to be detected, and a common illumination beampath is provided for these partial areas.
 4. The arrangement accordingto claim 1, wherein the reference structures are arranged so as to befixed in place on the exit face of the scanning prism, wherein separatepartial areas which do not intersect are reserved for the referencestructures and for the surface structures to be detected, and a commonillumination beam path is provided for these partial areas.
 5. Thearrangement according to claim 1, wherein the reference structures arearranged so as to be fixed in place on the image scanning surface of thescanning prism, wherein separate partial areas which do not intersectare reserved for the reference structures and for the surface structuresto be detected, and a common illumination beam path is provided forthese partial areas.
 6. The arrangement according to claim 1, whereinsemi-transparent reference structures are introduced in the illuminationbeam path and/or in the beam reflected by the image scanning surface. 7.The arrangement according to claim 6, wherein the semi-transparentreference structures are arranged on the scanning prism so as to befixed in place or are reflected into the beam path, wherein thecross-sectional areas of the beam path provided for the detection of thesurface structures and the cross-sectional areas of the beam pathprovided for the detection of the references structures and the surfaceportions of the reception surface provided for the detection of thesurface structures and the surface portions of the reception surfaceprovided for the detection of the reference structures intersect.
 8. Thearrangement according to claim 1, wherein the optoelectronic detectiondevice is combined with an evaluating circuit in which the images whichare obtained from the reference structures are subjected to a comparisonwith stored data, and conclusions concerning whether or not thepredetermined device parameters are adhered to can be drawn from thecomparison.
 9. The arrangement according to claim 6, wherein a firstcomputing function for determining a correction factor i_(ij) for everyimage element ij is provided in the evaluating circuit, wherek_(ij)=reference gray value/actual gray value, wherein every imageelement ij corresponds to an individual sensor of the reception surface,the reference gray values are determined for every individual sensorwith a uniformly bright image without structures and, further, a secondcomputing function is provided by which the determined actual gray valueof every image element ij is multiplied by the associated correctionfactor k_(ij), resulting in an image of the surface structure which isfree from reference structures.
 10. The arrangement according to claim6, wherein a control circuit is provided, by which the referencestructures are recorded and the recorded images are compared with thestored data whenever the apparatus is put into operation and/or afterexpiration of a predetermined operating period.
 11. The arrangementaccording to 6, claims wherein a control circuit is provided by whichthe detection of the reference structures is initiated manually andcomparison with the stored data can therefore be carried out at optionaltimes.
 12. The arrangement according to claim 2, wherein covers areprovided for the reference structures arranged on the image scanningsurface.
 13. The arrangement according to claim 1, wherein the referencestructures arranged on the scanning prism are printed, scratched in,etched, glued on in the form of a film or arranged in some otheroptically active way.
 14. The arrangement according to claim 1, whereinan optical device is provided for reflecting and/or mixing referencestructures into the illumination beam path or in the beam path reflectedby the image scanning surface.
 15. The arrangement according to claim 1,wherein grooves with different widths and different spacing andorientation are provided as reference marks and are configured formonitoring the image transfer, the MTF, the image scale, geometricdistortion, gray step linearity, gray value number and/orsignal-to-noise ration.